Explanation
The correct answer is C and D. Use a transit VPC with hub and spoke topology to create multiple VPN connections to the on-premises data center. Use the transit gateway attachment with VPN option to create multiple VPN connections to the on-premises data center.
According to the Fortinet documentation for Public Cloud Security, a transit VPC is a VPC that serves as a global network transit center for connecting multiple VPCs, remote networks, and virtual private networks (VPNs). A transit VPC can use a hub and spoke topology to create multiple VPN connections to the on-premises data center, using the FortiGate VM as a virtual appliance that provides network security and threat prevention.A transit VPC can also leverage Equal-Cost Multi-Path (ECMP) routing to achieve higher bandwidth and load balancing across multiple VPN tunnels1.
A transit gateway is a network transit hub that connects VPCs and on-premises networks. A transit gateway attachment is a resource that connects a VPC or VPN to a transit gateway. You can use the transit gateway attachment with VPN option to create multiple VPN connections to the on-premises data center, using the FortiGate VM as a virtual appliance that provides network security and threat prevention.A transit gateway attachment with VPN option can also leverage ECMP routing to achieve higher bandwidth and load balancing across multiple VPN tunnels2.
The other options are incorrect because:
Using ECMP and VPN to achieve higher bandwidth is not a complete solution, as it does not specify how to replace the existing VPC peering topology or how to connect the AWS VPCs to the on-premises data center.
Using transit VPC to build multiple VPC connections to the on-premises data center is not a correct solution, as it does not specify how to use a hub and spoke topology or how to leverage ECMP routing for higher bandwidth.
1:Fortinet Documentation Library - Transit VPC on AWS2:Fortinet Documentation Library - Deploying FortiGate VMs on AWS